N-Silylalkylamides and their use as adhesion promoters in room temperature vulcanizable polydiorganosiloxane compositions

ABSTRACT

N-Silylalkylamides are useful as adhesion promoters in room temperature vulcanizable compositions comprising polyalkoxysilyl-terminated polydiorganosiloxanes. The formamides, which are preferred, are novel compounds.

This invention relates to new compositions of matter, and moreparticularly to room temperature vulcanizable compositions andsilicon-nitrogen compounds useful as adhesion promoters therein.

Considerable attention has been directed in recent years to thedevelopment of improved one-package room temperature vulcanizable(hereinafter sometimes designated "RTV") compositions. Under idealconditions, these compositions would be stable for an indefinite periodwhen stored in the absence of moisture, and would promptly cure to atack-free elastomer upon contact with moisture, including the relativelysmall proportions of water vapor present in the atmosphere.

In a typical RTV composition, the predominant constituent is apolydiorganosiloxane (hereinafter sometimes designated "silicone" forbrevity) containing polyalkoxysilyl end groups, typicallydialkoxyalkylsilyl groups. Said end groups are capable of beingcrosslinked by atmospheric moisture in the presence of a suitablemetal-containing catalyst, usually an aluminum, titanium or tincompound. Disclosures of RTV compositions of this type are present inmany patents and publications.

A particularly useful one-package RTV composition is disclosed in U.S.Pat. No. 4,517,337. It employs a catalyst such as dibutyltinbis(acetylacetonate), which is stable in the presence of hydroxy speciessuch as methanol and silanol-terminated silicones and may therefore beemployed without scavengers for such hydroxy species.

Many RTV compositions have inadequate adhesion to various substrates,particularly metals such as aluminum and steel. Therefore, it is acommon practice to empldy adhesion promoters, typically varioussilicon-nitrogen compounds, in such compositions.

The present invention provides novel RTV compositions containingadhesion promoters, particularly scavenger-free RTV compositionscontaining tin complex catalysts such as dialkyltinbis(acetylacetonates). It also provides a class of novelsilicon-nitrogen compounds useful as adhesion promoters, and a methodfor their preparation.

In one of its aspects, the present invention includes a method forimproving adhesion to a substrate of a room temperature vulcanizablecomposition comprising a polyalkoxy-terminated polydiorganosiloxane(silicone) and a catalytic amount of a curing catalyst, said methodcomprising incorporating therein an effective amount, typically about0.1-5.0 parts per 100 parts of said polyalkoxysilyl-terminatedpolydiorganosiloxane, of an N-silylalkylamide compound having theformula ##STR1## wherein R¹ is hydrogen or an alkyl radical containingfrom 1 to about 8 carbon atoms, each R² is independently hydrogen or aprimary or secondary alkyl containing from 1 to about 4 carbon atoms, R³is an alkyl containing from 1 to about 8 carbon atoms and n is from 2 toabout 8.

Another aspect of the invention is RTV compositions comprising (A) atleast one polyalkoxy-terminated silicone, (B) a catalytic amount of acuring catalyst, and (C) an amount effective to enhance adhesion to asubstrate of said N-silylalkylamide.

In the N-silylalkylamides of formula I, R¹ may be hydrogen or an alkylgroup, most often a primary or secondary alkyl group. The formamides, inwhich R¹ is hydrogen, are preferred and are novel compounds.Accordingly, another aspect of the invention is N-silylalkylformamidesof formula I wherein R¹ is hydrogen.

The R² radicals may be hydrogen or alkyl radicals as indicated and areusually hydrogen. R³ is an alkyl radical, usually methyl. The value of nis from 2 to about 8 and is preferably 2 or 3, especially 3.

Certain N-silylalkylamides used according to the invention, such asN-(3-trimethoxysilylpropyl)acetamide, are known compounds. Reference ismade, for example, to U.S. Pat. No. 3,919,276 for their preparation.

The N-silylalkylformamides may be prepared by the reaction of an alkylalkanoate having the formula ##STR2## with an aminoalkylsilane of theformula ##STR3## wherein R¹⁻³ and n are as previously defined and R⁴ isC₁₋₄ alkyl and especially methyl. This reaction is most convenientlyconducted by heating a mixture of the two reagents at a temperature ofabout 30°-80° C., preferably in an inert atmosphere such as nitrogen,with removal of the by-product R⁴ OH by distillation. A solvent may beemployed but is seldom necessary. Most often, the alkyl alkanoate isused in excess, typically in a molar ratio of about 5-10:1. When thereaction is complete, conventional methods of product isolation may beused where necessary. However, the preparation of formamides by the useof such low boiling esters as methyl formate produces only volatileby-products and purification operations may then be unnecessary.

The polyalkoxysilyl-terminated silicones useful as component A in theRTV compositions of this invention may be represented by the formula##STR4## wherein R⁵ is an unsubstituted or substituted hydrocarbonradical containing from 1 to about 13 carbon atoms, R⁶ is an alkyl,alkoxyalkyl, acylalkyl, acyloxyalkyl or cyanoalkyl radical containingfrom 1 to about 8 carbon atoms or an aralkyl radical containing from 1to about 14 carbon atoms, each R⁷ is independently an unsubstituted orsubstituted hydrocarbon radical containing about 1-13 carbon atoms, a is0 or 1 and m is in the range of about 5-5000. Illustrative R⁵ and R⁷radicals are methyl, ethyl, phenyl, trifluoropropyl and vinyl. Alkylradicals having up to about 4 carbon atoms and especially methylradicals are preferred. R⁶ may be alkyl or the designated substitutedalkyl radicals containing aryl, ethyl, ester, ketone or cyanosubstituents; it is also most often C₁₋₄ alkyl and especially methyl.The value of a is 0 or 1 and most often 1.

Component A may be prepared in situ in the RTV composition by thereaction of a silanol-terminated silicone and, as an endcapping reagent,a polyalkoxysilane such as methyltrimethoxysilane, in accordance withU.S. Pat. No. 4,395,526. It may also be previously formed by the samereaction, most often in the presence of a catalyst as disclosed, forexample, in U.S. Pat. No. 4,515,932 and copending, commonly ownedapplication Ser. No. 90,183 filed Aug. 27, 1987. The time of itsformation is not critical for the purposes of this invention, althoughit is frequently preferred to employ a previously formedpolyalkoxylsilyl-terminated silicone.

Component B, the curing catalyst, may be any of the metal-containingcatalysts known in the art. As previously noted, these are usuallyaluminum, titanium or tin compounds. In a particularly preferredembodiment of the invention, the catalyst is an organotin complex of theformula ##STR5## wherein R⁸ is an unsubstituted or substitutedhydrocarbon radical containing from 1 to about 18 carbon atoms; each ofR⁹, R¹⁰ and R¹¹ is independently hydrogen, R¹², Si(R¹²)₃, acyl ornitrile; and R¹² is an unsubstituted or substituted hydrocarbon radicalcontaining from 1 to about 18 carbon atoms. Compositions cnntaining suchcomplexes, especially di-n-butyltin bis(acetylacetonate), are disclosedand claimed in the aforementioned U.S. Pat. No. 4,517,337, thedisclosure of which is incorporated by reference herein.

Component C, the N-silylalkylamide, is employed in the RTV compositionas an adhesion promoter. Its action as such is frequently optimized ifit is employed in combination with (D) a cyanoalkyltrialkoxysilane, mostoften 2-cyanoethyltrimethoxysilane (hereinafter "CETMS") or3-cyanopropyltrimethoxysilane, which acts as a synergist therefor.

Components B and C are present in the RTV compositions of this inventionin effective proportions to serve as catalysts and adhesion promoters,respectively. In general, about 0.1-10.0 parts of component B and about0.1-5.0 parts of component C are employed per 100 parts of component A.When employed, component D is usually present in the amount of about0.1-5.0 parts per 100 parts of component A.

The RTV compositions of the invention may also contain otherconstituents in common use in such compositions, including curingcatalyst accelerators, scavengers for hydroxy species, plasticizers,pigments and fillers. In particular, at least one of the following maybe present, all proportions being per 100 parts of component A:

(E) about 0.05-5.0 parts of a diketone of the formula ##STR6## whereinR⁹⁻¹¹ are as previously defined;

(F) about 0.01-10.0 parts of at least one polyalkoxysilane of theformula

    (R.sup.5).sub.a Si(OR.sup.6).sub.4-a                       (V)

wherein R⁵⁻⁶ and a are as previously defined;

(G) about 1-50 parts of a plasticizer;

(H) about 5-700 parts of at least one filler; and

(J) about 0.1-5.0 parts of an amine or guanidine as a curingaccelerator.

Components E and F are often particularly preferred other constituents.Their presence contributes to the shelf stability of the RTV compositionin the absence of moisture and its rapidity of cure in the presence ofmoisture.

The presence of component G is also frequently preferred. Suitableplasticizers useful as component G include trialkylsilyl-terminatedpolydiorganosiloxanes of the formula ##STR7## wherein R⁷ is aspreviously defined and p is in the range of about 25-5000.

The presence or absence of component H, the filler, will depend to someextent on the intended use of the RTV composition. When the compositionis to be used as a construction sealant or caulking compound, relativelylarge proportions of filler may be employed. For other uses, minorproportions of filler or no filler may be advisable. Suitable fillersinclude reinforcing materials such as silica aerogel, fumed silica,precipitated silica, glass fibers, titanium dioxide, zirconium silicate,iron oxide, calcium carbonate, diatomaceous earth and carbon black, andextending materials such as ground quartz and polyvinyl chloride, aswell as mixtures thereof. It is frequently advantageous to pretreat asilica filler with an activating agent such asoctamethylcyclotetrasiloxane.

Various amines and guanidines, optionally alkoxysilyl-substituted, areknown to be useful as curing accelerators (component J). Suitableaccelerators are disclosed, for example, in the aforementioned U.S. Pat.No. 4,517,337.

The preparation and properties of the compositions of the presentinvention is illustrated by the following examples. Parts are by weight.All RTV compositions were prepared by conventional high-shear mixingtechniques in a nitrogen atmosphere, under the equivalent of "dry box"conditions. Viscosities are Brookfield viscosities at 25° C..

EXAMPLE 1

A mixture of 7 parts (39 mmol.) of 3-aminopropyltrimethoxysilane and 20parts (330 mmol.) of methyl formate was heated under reflux in anitrogen atmosphere. The reaction was monitored by gas chromatographyand was found to be complete after 3 hours. Volatiles were then removedin vacuum, yielding the desired N-(3-trimethoxysilylpropyl)formamide.

EXAMPLE 2

An RTV composition was prepared by initially blending 100 parts of apolymethoxysilyl-terminated polydimethylsiloxane having a viscosity of300 poises, 1 part of CETMS, 16 parts ofoctamethylcyclotetrasiloxane-treated fumed silica and 23 parts of atrimethylsilyl-terminated polydimethylsiloxane oil, and subsequentlyadding 0.42 part of dibutyltin bis(acetylacetonate), 0.28 part ofacetylacetone, 0.35 part of methyltrimethoxysilane and 0.7 part of theproduct of Example 1. Adhesion-in-peel on unprimed aluminum wasdetermined according to ASTM test method C794 after 7 days of cure at50% relative humidity. Comparison was made with a control in which 0.7part of glycidoxypropyltrimethoxysilane was substituted for the productof Example 1.

The product of Example 1 gave an adhesion value of 179.3 millipascals,while the control gave a value less than 35 millipascals.

What is claimed is:
 1. An N-silylalkylformamide compound having theformula ##STR8## wherein each R² is independently hydrogen or a primaryor secondary alkyl radical containing from 1 to about 4 carbon atoms, R³is an alkyl radical containing from 1 to about 8 carbon atoms and n isfrom 2 to about
 8. 2. A compound according to claim 1 wherein each R² ishydrogen.
 3. A compound according to claim 1 wherein R³ is methyl and nis
 3. 4. A compound according to claim 19 wherein R⁵⁻⁷, R⁹ and R¹¹ aremethyl, R⁸ is n-butyl and R¹⁰ is hydrogen.